1. Field of the Invention
The present invention relates to a broadband wireless access (BWA) communication system, and more particularly to a system and method for transmitting and receiving information about hybrid automatic repeat request (HARQ) buffer capability of a mobile station (MS).
2. Description of the Related Art
Fourth generation (4G) communication systems (the next generation of communication systems) are being designed to provide users with services having various qualities of service (QoSs) with a high transmission speed. Particularly, in current 4G communication systems, research is actively being conducted to develop a new type of communication system for ensuring mobility and QoS in a broadband wireless access (BWA) communication system, such as a wireless local area network (LAN) and a wireless metropolitan area network (MAN) system, in order to support high speed services. The representative communication system is the IEEE (Institute of Electrical and Electronics Engineers) 802.16d/e communication system.
The IEEE 802.16d/e communication system employs an orthogonal frequency division multiplexing/orthogonal frequency division multiple access (OFDM/OFDMA) scheme in order to enable a physical channel of the wireless MAN system to support a broadband transmission network. The structure of the IEEE 802.16e communication system will now be described with reference to FIG. 1, which schematically illustrates the structure of a typical IEEE 802.16e communication system.
The IEEE 802.16e communication system has a multi-cell structure, that is, for example, a cell 100 and a cell 150. In addition, the IEEE 802.16e communication system includes a base station (BS) 110 managing the cell 100, a base station (BS) 140 managing the cell 150, and a plurality of mobile stations (MSs) 111, 113, 130, 151, and 153. The transmission and reception of signals between the base stations 110 and 140 and the MSs 111, 113, 130, 151 and 153 are executed using the OFDM/OFDMA scheme.
Meanwhile, during data transmission, inevitable errors caused by noise, interference, and fading occur according to channel conditions, thereby causing information loss. In order to reduce this information loss, various error-control schemes are used according to the characteristics of channels to increase system reliability. A representative error-control scheme is a hybrid automatic repeat request (HARQ) scheme. The HARQ scheme is a new error-control scheme which is produced by combining the advantages of an automatic retransmission request (ARQ) scheme and a forward error correction (FEC) scheme.
According to the HARQ scheme, an acknowledgment/non-acknowledgment (ACK/NACK) signal in response to a signal transmitted from a transmitter is fedback from a receiver to the transmitter, and the transmitter retransmits the transmitted signal when receiving the NACK signal from the receiver, thereby improving the reliability of transmission. The receiver feeds the ACK signal back to the transmitter when receiving a signal without error transmitted from the transmitter, and the receiver feeds the NACK signal back to the transmitter when receiving an abnormal signal from the transmitter, that is, when there is an error in the signal transmitted from the transmitter. In this case, when the HARQ scheme is employed, a transmitted signal has a unit of an HARQ encoder packet (Hep). One Hep is generated by inserting a cyclic redundancy check (CRC) code into one Medium Access Control-Protocol Data Unit (MAC-PDU) or a plurality of MAC-PDUs concatenated with each other.
Currently, the IEEE 802.16d/e system supports two types of HARQ schemes, that is a first type (type-I) HARQ scheme and a second type (type-II) HARQ scheme. In the following description, the two types of HARQ schemes will be described.
According to the type-I HARQ scheme, which is also called a “chase combining (CC)” scheme, a transmitter transmits signals of the same format upon initial transmission and retransmission. Then, a receiver receives signals transmitted from the transmitter upon the initial transmission and the retransmission, soft-combines two received signals, and then decodes the soft-combined signals.
According to the type-II HARQ scheme, which is also called a “incremental redundancy (IR)” scheme, a transmitter transmits signals of different formats upon initial transmission and retransmission. Then, a receiver code-combines signals transmitted from the transmitter upon the initial transmission and the retransmission, and then decodes the code-combined signals.
As described above, the IEEE802.16d/e communication system supports two types of HARQ schemes. In order to realize the HARQ scheme, it is necessary to exchange parameters between an MS and a base station (BS) for the HARQ scheme when the MS performs an initialization or handover operation. However, in the current IEEE 802.16d/e communication system, parameters exchanged for the HARQ scheme operation upon the initialization or handover of an MS does not include any parameter relating to the buffer capability of the MS.
When the HARQ scheme operates without regard to the buffer capability of the MS, the following problems occur:
First, since a base station does not know the buffer capability of an MS at all, the base station may determine a size of Hep beyond the capacity of the MS. In this case, it is impossible for the MS to receive the Hep transmitted from the base station, thereby causing unnecessary retransmission due to the abnormal reception of the Hep.
Second, an MS must have not only a first memory capacity required for the combining operation with respect to erroneous Heps, but also a second memory capacity required for reordering MAC-PDUs, which have been generated by decoding normal Heps, in order to transmit the MAC-PDUs to an upper layer. When the MS does not secure the memory capacity required for the combining and reordering, it is impossible for the MS to receive a Hep transmitted from the base station, thereby causing unnecessary retransmission due to abnormal reception of the Hep.
As described above, when the HARQ scheme operates without regard to the buffer capability of an MS, an unnecessary retransmission of a Hep occurs. Such an unnecessary retransmission of an Hep causes traffic delay and unnecessary resource allocation, thereby degrading the performance of the entire system.